1. Field of the Invention
This invention relates to a nonaqueous electrolyte secondary battery, particularly being highly reliable, and having an improved negative electrode, high energy density, large capacity, and not suffering from short circuiting due to growing of dendrite.
2. Description of the Prior Art
Nonaqueous electrolyte secondary batteries provided with negative electrodes of alkaline metal such as lithium(Li) or sodium(Na) have high electromotive force and are expected to realize higher energy density compared with conventional nickel-cadmium or lead batteries. Nonaqueous electrolyte secondary batteries, in particular having negative electrodes of lithium, have been investigated.
However, batteries with metal alkaline as the negative electrode experience dendrite growth while being charged. This often results in short circuiting and low reliability.
In order to solve this problem, a negative electrode of alloy consisting of Li and aluminum(Al) and lead(Pb) has been investigated. With this negative electrode of alloy, Li is occluded in this negative alloy-electrode while charging, and dendrite does not grow, thus resulting in a highly reliable battery. However, because the discharging potential of this negative alloy-electrode has a potential which is about 0.5 V higher than the metal Li, the voltage of the battery decreases 0.5 V. Accordingly, the energy density of the battery is also lowered.
On the other hand, application of an intercalation compound (e.g. graphite) between Li and carbon(C) as the active material of the negative electrode has been investigated. In this case, too, dendrite does not grow as a result of charging, since Li goes into the parts between the carbon layers. The negative electrode, with the discharging potential about 0.1 V, which has a higher potential than metal Li and a small decrease of the battery voltage, is a desirable one. However, it still has a serious problem. The penetration of Li into the interlayer part is limited, theoretically, to the quantity decided by formula C.sub.6 Li at the maximum, or a discharge capacity of 372 Ah/kg. For the actual batteries, the electric capacity for charging and discharging is as small as 230 Ah/kg.
Also, an electrode material consisting of BCN(H) has been proposed (Japanese Patent Gazette No. Toku.Kai.Hei 3-165463), in which a uniform compound consisting of elements B, C and N and/or those added with hydrogen is used as an electrode material. Of this BCN(H), the discharging potential has a potential about 0.5 V higher than that of metal Li, and the discharging electric quantity for unit active material of the negative electrode is as small as 100 mAh/g.